Blender 2.8 Beta - Unknown Rectangle when camera is selected
That is the Render Border. When rendered shading is active, the render will be constrained to that portion of the screen. You can disable it with CtrlAltB, or you can enable and/or draw a new one with CtrlB.

See Render Border&#39;s related check boxes - How do they work? for a more complete overview of its functionality.